This invention relates to a filterbank structure and a method for filtering and separating an information signal into different bands, particularly for such filtering and separation of audio signals in hearing aids. This invention more particularly relates to such a technique carried out using digital signal processing in hearing aids. This invention more particularly relates to a method and architecture for a digital filterbank for hearing aid applications.
Hearing loss is generally associated with a loss of hearing sensitivity which is a function of frequency. The most common type of sensitivity loss is an increasing function of frequency. Sensitivity is typically a function of speech level as well. Hence, loud sounds should be amplified less than soft sounds. It has been long known that a hearing aid should treat the various frequency components of speech differently to render them intelligible to a hearing impaired person.
Known analog hearing aids use relatively simple methods to alter their frequency shaping and dynamic range compression to mitigate the loss in hearing sensitivity for frequency and level.
Digital techniques promise far greater possibilities for signal processing to aid the hearing impaired. The present inventors have realized that digital filterbanks offer a flexible framework for separating the input signal into a number of independent frequency bands, for separate or combined processing. This allows great processing flexibility as the bands can be treated independently to compensate more precisely for hearing loss.
In accordance with the present invention, there is provided a filterbank for filtering an information signal, the filterbank structure comprising a filter means defining a filter bandwidth, said filter means filtering said audio signal and separating said audio signal into a plurality of frequency band signals each representing one of a plurality of uniformly spaced frequency bands within said filter bandwidth, wherein the filter means includes a selection input enabling the number of frequency band signals and the bandwidth of each frequency band to be selected.
It is to be appreciated that while it is envisaged that the number of frequency bands and their bandwidth will usually be parameters that can be adjustable by the selection input, this is not always the case. More generally, the filterbank can be configured to enable one or more of usual parameters of a digital filterbank to be adjustable, and these can include: the number of bands; the width of each band; whether the bands have abutting band edges, overlap or are spaced apart; coefficients for both analysis and synthesis windows; whether there is any relationship between the analysis and synthesis windows; even or odd stacking of bands; and the degree of oversampling above the critical sampling rate. Details of these parameters are set out below.
Preferably, the filter means comprises (a) a first analysis filterbank means for separating said signal into a plurality of N separate frequency band signals; (b) processing means for receiving and processing each of said separate frequency band signals to provide N separate processed frequency band signals; and (c) a second synthesis filterbank means for receiving and recombining the N separate processed frequency band signals into a single output signal, wherein both of the first analysis filterbank means and the second synthesis filterbank means are connected to the selection input, to enable the number of bands and the band width of each frequency band to be selected, the processing means being coupled between the first analysis filterbank means and the second synthesis filterbank means.
In another aspect of the invention, there is provided a filterbank for filtering an information signal, the filterbank structure comprising a filter means defining a filter bandwidth, the filter means filtering the information signal and separating the information signal into a plurality of frequency band signals each representing one of a plurality of uniformly spaced frequency bands within the filter bandwidth; wherein the filter means comprises: (a) a first analysis filterbank means for separating the information signal into a plurality of N separate frequency band signals; (b) a processing means for receiving and processing each of the separate frequency band signals to provide N separate processed frequency band signals; and (c) a second synthesis filterbank means for receiving and recombining the N separate processed frequency band signals into a single output signal. The first analysis filterbank means, the processing means and the second synthesis filterbank means utilize digital signal processing. The first analysis filterbank means receives an input digital sample stream and the second synthesis filterbank means provides an output digital data stream as the output signal. Furthermore, the filterbank circuit includes an analog-to-digital conversion means connected to the first analysis filterbank for receiving the information signal and for converting the information signal into the input digital sample stream at an initial input sampling rate which forms the information signal for the analysis filterbank, and a digital-to-analog conversion means connected to the second synthesis filterbank for converting the output digital data stream to form an analog version of the single output signal. In addition, the first analysis filterbank means comprises: (a) a blocking means for receiving the input digital sample stream and blocking a first number, R where R ~ N, of the digital samples so as to provide a blocked input digital sample stream, the ratio of N/R corresponding to an oversampling factor; (b) an analysis window means for applying an analysis window function to the input digital sample stream to provide a windowed blocked digital sample stream, the analysis window function being defined by a set of analysis window coefficients; (c) a time folding means for overlapping and adding blocks of the windowed blocked digital sample stream, each of the blocks comprising N digital samples, to provide a summed block of N digital samples; and (d) a discrete transform means for receiving the summed block of N digital samples and transforming the N digital samples into a discrete frequency domain signal having N components, the N components corresponding to the N frequency band signals.
The filterbank may be adapted to receive a single real monaural information signal, wherein said transform means generates non-negative frequency band signals and negative frequency band signals, said negative frequency band signals being derivable from the non-negative frequency band signals, and said processing means processes only said non-negative frequency band signals. Alternatively is adapted to filter an audio signal comprising first and second real monaural information signals which are combined into a complex stereo signal and wherein said transform means generates N combined frequency band signals, and wherein said processing means includes: (a) channel separation means for separating the N combined frequency band signals into the N frequency band signals corresponding to said first information signal and the N frequency band signals corresponding to said second information signal, each of said N frequency band signals comprising non-negative and negative frequency band signals; (b) first independent channel processing means connected to the channel separation means for receiving and processing each of said separate frequency band signals of said first information signal to provide a first set of N separate processed frequency band signals; (c) second independent channel processing means connected to channel separation means for receiving and processing each of said separate frequency band signals of said second information signal to provide a second set of N separate processed frequency band signals; and (d) channel combination means connected to the first and second independent channel processing means for combining said first set of N processed separate frequency band signals and said second set of N processed separate frequency band signals.
In accordance with another aspect of the present invention, there is provided a method of processing an information signal to selectively modify different frequency bands, the method comprising the steps of: (1) defining a frequency bandwidth to be analyzed; (2) dividing the frequency bandwidth into a plurality of uniformly spaced bands and defining characteristics of the filter bands; (3) filtering the information signal to separate the signal into a plurality of frequency band signals, each representing one of said uniform filter bands; (4) processing the frequency band signals; and (5) recombining the signals of the individual bands to form an output signal. Preferably, step (4) comprises setting a gain for each frequency band and multiplying each frequency band signal by the respective set gain. Also preferably, the method further comprises: (a) in step (3) separating said signal into N separate frequency band signals; (b) in step (4) processing each of said separate frequency band signals to provide N separate processed frequency band signals; (c) in step (5), recombining the N separate processed frequency band signals to form the output signal; and (d) selecting the number of bands and the bandwidth of each frequency band.
In another aspect the method includes transforming the information signal into the frequency domain, providing N separate frequency band signals in the frequency domain, and effecting an inverse transform of the N separate processed frequency band signals into the output signal in the time domain.
The signal, in one variant of the invention, is filtered to give a plurality of evenly stacked bands, as described in Crochiere, R. E. and Rabiner, L. R., Multirate Digital Signal Processing, (Prentice-Hall, 1988) which is incorporated herein by reference. Alternatively, it can be filtered to give a plurality of oddly stacked bands. This has the advantage that the placement of the band edges is selectable and this technique gives twice the number of potential band edges. The band edges can be selected depending on the characteristics of a person""s hearing loss. In further variants of the invention, other parameters of a digital filterbank are made adjustable either alone or in combination.